Field of Invention
The present invention relates generally to electrical circuits for voltage control, and more particularly, to digital circuits for voltage control, and hence delivery of power, to electrical loads.
Description of the Related Art
Electrical devices and appliances are generally designed to operate at specific power supplies in terms of voltage magnitude and frequency, and other properties. When in use, any deviation from the specified powering conditions could render the devices or appliances inefficient, inoperative or even permanently defective.
Therefore, ever since human deployment of electricity, it is a common goal of electrical and electronics engineers and scientists to develop devices and methods to control and deliver electrical power to the loads efficiently. Various inverters, converters, voltage regulators, power amplifying and power switching components, electrical sensors, etc. are invented and developed to increase the capabilities in electrical power control, in terms of energy efficiency, control accuracy, response speed, power level and system cost, etc.
In practice, electrical voltage control techniques are employed to control electrical parameters other than voltages. For example, by controlling the voltage applied to a constant impedance device, current through the device is controlled. As another example, by controlling the voltage applied to a load, the power generated by the load is controlled. Thus in practice electrical voltage control apparatus functions in many different forms and in many different application areas, such as:    1 Variable voltage supplies, such as those as voltage references for calibration and testing    2 Variable current supplies, such as those as current references for calibration and testing    3 Voltage regulators, such as mains voltage regulators for powering electrical appliances    4 Current regulators, such as those for powering LED lamps    5 Power regulators, such as those for thermal control
Broadly speaking, we may define two distinct methods in voltage control, namely analogue and digital approaches. By analogue approach, voltages are scaled up or down continuously through any voltage levels within the control range. By digital approach, to be called Digital Voltage Control throughout this specification, voltage levels are “stepped” through discrete levels within the control range. One common way is, as often used for AC mains regulation, through switching in and out of transformer coils as “voltage cells”, i.e. voltage sources which are in galvanic-isolation from each other.
There are a number of merits of digital approach over the analogue approach. The analogue approach is through linear control of active electronic devices such as transistors operating in the linear mode, or through circuit switching by active electronic devices such as transistors operating in the switching mode. By linear mode of operation, voltage control can be achieved with high control accuracy and high control speed but at the cost of low power efficiency. By switching mode of operation, voltage control can be achieved with high power efficiency but often with compromised control accuracy and control speed. For very high power applications, the analogue approach, either in linear or in switching mode, faces the difficulties of very high cost or unavailability of suitable active high power or high frequency devices. Further there are more EMI and EMC issues in association with high power and high frequency switching.
The digital approach as adopted by present invention is through switching in and out of “voltage cells” at the usually low power frequency (such as the 50 or 60 Hz mains frequency, or even DC) of the voltage under control, rather than at very high frequencies. Demand on switching speed of the switching devices as well as on the control schemes are not high in general, even at very high power levels. Further, as switching is performed at low frequencies, the issue on EMI or EMC is relatively less serious and might be more easily handled. The digital approach is therefore a better choice to the analogue approach when power-handling capacity and low cost are the prime considerations. Moreover, since the switching loss at low frequency is relatively low, the digital approach enjoys also the benefits of high power efficiency. Furthermore, that no and little distortion is introduced through switching is yet another advantage by the digital approach as compared to the analogue approach.
However, there is still a very important aspect of voltage control or regulation to be considered, namely the accuracy of control. Since by the digital approach, the voltage is varied by steps, the accuracy of control is always limited by the size of the voltage steps. It is obvious for a fixed range of voltage control, the fineness of control is inversely proportional to the number of voltage levels that could be “stepped” through. It is also obvious that for a fixed number of voltage levels, all voltage steps should be made equal to achieve the highest accuracy of control.
When the number of steps is increased for the purpose of achieving finer control, the number of switches required will inevitably increase. Since the switches are the key and relatively expensive components of the system, accuracy of control has often been compromised for lowering the system cost by limiting the number of switches deployed. This is highly undesirable and many different varieties of switching circuit topologies and control methods have been attempted in the past to achieve higher control accuracy while limiting the number of switches employed for circuit simplicity and cost reduction. However these existing designs are in general complicated in overall system structure, restrictive in deployment and often overly complicated in control methodology.
Further, when fine steps are achieved for high control accuracy, a new challenge of maintaining system stability will be in front of the designer. Dependent of the actual circuit design and the accuracy in circuit implementation, monotonicity between the digital control signal and the controlled step voltage output would be lost as the size of the steps decreases to some extent. Consequently, lack of monotonicity causes system instability and also reduction in control accuracy.
While piecemeal improvements or alterations are revealed in many prior inventions, none has actually proposed a unified approach to address the above issues. The present invention is intended to solve all these problems and it will become clear when the invention is disclosed herewith exemplary embodiments.
Prior arts in the voltage control or regulation are found typically in AC voltage regulators, whereby many methods and devices are developed to control the AC voltage through digital approach, and some are revealed by the following patents:                CN201149665        CN201251718        CN201281825        CN201805273        CN201984364        CN201984365        CN201984366        GB1300229        GB2324389        U.S. Pat. No. 3,970,918        U.S. Pat. No. 4,178,539        U.S. Pat. No. 4,716,357        U.S. Pat. No. 4,896,092        U.S. Pat. No. 5,545,971        U.S. Pat. No. 5,932,997        U.S. Pat. No. 6,137,277        U.S. Pat. No. 6,417,651        U.S. Pat. No. 7,816,894        U.S. Pat. No. 7,800,349        US20110043182        US20110273149        
In majority of the above inventions and disclosed embodiments, the circuit topologies proposed tend to be very specific and hence very restrictive. The restrictiveness in circuit topologies has presented difficulties to the designer in optimizing the performance of the voltage regulator under practical considerations, such as the difficulty in deciding the best number of voltage modules, the best number of voltage cells in each voltage module (such as the number and turns of transformer coils in the design of transformers for tap-switching voltage regulators), the best number of switches in each voltage module, the most suitable control methodologies and control modules, etc. Consequently, there is a lack of design flexibility for optimizing the performance of the voltage regulator in terms of accuracy of control, voltage range of control, speed of response, cost of implementation, and cost of maintenance, etc.
Further, linearity and monotonicity of the voltage variation are not generally addressed. In many of the inventions, the equal voltage steps are not achieved or not even intended to be achieved. The step sizes are simply not constant by design in these inventions. The result is that the voltage change is non-linear or even worse, not monotonic. Non-linearity will lower the control accuracy achievable, while non-monotonicity will render a feedback control system unstable. Both are detrimental to the performance of the digital voltage control system.
Further still, none of the prior inventions has addressed the issues on the practical limitations affecting the linearity and monotonicity of the voltage under digital control. Consequently the performance of the digital voltage control system, in terms of control accuracy and system stability, is likely compromised due to the oversight of this aspect in system design.
In most cases, prior art designs fail to show the ideal or the preferred theoretical ratios of the voltage cells. In a number of cases, some ratios are proposed without any reasoning as how these ratios are arrived at. Consequently there is no guidance in design to optimize the system, in terms of control accuracy and control range, through proper selection by design the number and magnitude of the voltage cells, and the voltage ratios between the voltage cells.
As will be clear from the following detailed description, the present invention adopts a unified approach to address the above issues not sufficiently addressed before. Apart from practical limitations of components available, there is no restriction by the present disclosed approach in designing the voltage controller in terms of control accuracy, number of switches deployed, number and magnitude of voltage cells. The method of control and the associated control circuitry is simple and straight forward, while the practical limitations affecting the linearity and monotonicity will be addressed to have its consequent bad effects removed too. This will be explained in details with disclosed embodiments for illustration.
Despite that above quoted prior arts dual with transformer tap-switching voltage regulators or controllers, whereby independent transformer coils are depicted as voltage cells in galvanic-isolation, the present invention applies to any other electrical voltage sources in any forms (named “voltage cells” throughout this patent specification) such as these quoted below as examples:                Electrochemical battery cells        Solar cells        Fuel cells        Thermopiles        Power transformers energized by supply voltage        Electricity generators        
Furthermore, by the duality property of electrical circuits, the present invention can be applied also to electrical current control, as will be explained in more details.